Saturday, 12 March 2016

The Big List of Propulsion Failures

When writing a science fiction story, often the author will place the characters inside a spaceship heading towards danger or adventure. For dramatic tension, the reactor explodes! The engines explode! The propellant tanks leak... the fuel explodes!

You have created a vast world, written colourful characters and captivating moments, so why the lack imagination when it comes to failure modes? Hopefully, this post will inspire you to rewrite those dramatic moments.

The number of configurations and technologies involved in propulsion is endless. Within the setting, just like in real life, a single technology can have hundreds, if not thousands of parts working together in different ways, depending on brand, manufacturer, design ideology or local constraints and so on...

To limit this post's length (and your reading time) to humane levels, we'll go down an alphabetical list of drive systems and look at how they can fail (and the consequences) in varying levels of detail.

The larger the antimatter particle, the more easily is interacts with normal matter.

Ah, the big, scary reverse of matter. It contains 10 billion times more energy than gasoline, and a 1000 times more than uranium. Understandably, even a few grams of this stuff can blow up the largest spaceships.Since nothing can touch antimatter without getting annihilated, it has to be handled by magnetic fields. The most common configuration for storing antimatter as a fuel is magnetic bottles. These create the first point of failure.

The current way of trapping antimatter.

If the magnets are damaged, lose power or (if they're superconductive) get too hot, they'll stop producing stable magnetic fields. The antimatter drifts inside the magnetic bottles and touches the walls.

Antimatter-matter annihilation produces high energy photons. Basically, gamma rays. These radiate out of the point of annihilation, are absorbed by the bottle's casing, and are re-released as heat. This all happens in less than nanoseconds. This heat, much like a nuclear detonation, vaporizes its surroundings and blows everything away.

A containment failure will be felt as a single large explosion, breaking the ship in two and leaving behind a rapidly expanding cloud of metal vapor and leaking propellant.

Magnetic bottles are supposed to be durable, insulating and have power backups. This means, if they're not placed idiotically next each other like eggs in a basket, a fraction of them will be blown away instead of instantly vaporized. The damaged, hot containers eventually run out of power and produce their own nasty bursts of gamma rays, with nothing to absorb and convert it into heat.

The visual effect is like a firework - a big flash, expanding smoke and trails of fizzling secondary explosions.

The brightness would be much harsher, and linger for less.

Antimatter, if it is to be used as a power source, has to be released in a controlled manner from these bottles. This would be done by weakening one end of the magnetic fields so that a steady rate of antimatter particles filter through. These would then be accelerated and ejected into the reaction chamber configuration of choice.

This would be a second point of failure.

Everything depends on magnets, so has the same vulnerabilities.

If magnetic containment failed once the antimatter particles are ejected from containment, the effects would vary from catastrophic to mild. If the failure happens near the bottle, the resultant annihilations would affect the surrounding magnets, or create gamma rays that penetrate through the containment shielding and heat up the stored animatter inside, causing it to jump around and touch the inner walls. This would set off a rapid chain of events that leads to a vaporized spaceship.

If the failure happened somewhere midway, it depends of how much antimatter escapes. If you have a low-power rocket engine that only sips antimatter, very few particles would be in the fuel lines at any one time. They'd damage the transport assembly, and probably lead to a prompt shutdown of all propulsion systems. You'd then have to send engineers or robots to repair the damage, like a SciFi version of mechanics repairing a steam pipe explosion.

Some failure modes involve no damage to the ship itself. The 'nozzle' assembly injects a stream of anti-particles into the propellant, such as a supply of hydrogen. What if the hydrogen supply gets cut off? Antimatter would get ejected straight out of the nozzle. This could have dire effects in the vicinity of other spacecraft, or in an orbit where the antimatter would eventually hit another vehicle.

Instead of spreading out harmlessly like hot exhaust gasses, it'd strike a spacecraft's hull and release bursts of deadly radiation, or eat through it like acid. Hot, explodey acid.

In narrative terms, the characters will be permanently stress-sweating whenever a sensor detects elevated temperatures or increased radiation levels. A stray gamma ray could be cosmic... or the signs of an anti-particle leak. All this radiation would also affect the materials that make up the nozzle become weaker over time. If the nozzle and the nearby magnets are not regularly replaced, they could shatter and blow off the rear end of the spaceship....

Beamed Power

Why drag along a reactor or rocket engine when you can have your propulsion delivered to you? This is the basic concept behind beamed power.

The main variants rely on lasers of various wavelengths either focused and used to heat up onboard propellant, or reflected out into space, using the momentum of photons as propulsion.

Having a giant laser beam pointed at the back of your spaceship creates an obvious failure mode: misalignment.If you rely on very powerful beams, having it hit something else than a reflective mirror (or that mirror at a bad angle) will turn that beam into a weapon that destroys the spaceship. This could be the result of defects in the laser generator, the focusing assembly, a miscommunication of your position, or a damaged spaceship tumbling and pointing the wrong way into the beam.

Laser thermal rocket.

In practice, this shouldn't be too worrying. Feedback mechanisms should be in place to detect that the laser isn't hitting the right spot, or that the target is emitting the wrong type of heat. Also, the beam would likely have a 'safety' mode, used to gauge the distance and orientation of the target before turning up the power.

For a laser-thermal rocket engine, catching the beam correctly is only part of the crew's worries. The mirrors that refocus the beam onto the propellant in the reaction chamber turn the propulsion laser into a focused stream of death. If the focusing mirrors are bent, heated up and warped, or simply controlled incorrectly, the focused beam would burn through the reaction chamber, cut propellant lines, damage pumps or in the worst cases, burn up and melt away, rendering the propulsion useless until the damaged parts are replaced.

Another concern is the cooling system for the mirrors handling the concentrated beam. Even a 99.99% reflective surface absorbs a fraction of the incoming laser. If the beam energy is in the megawatts, then you'll have to dissipate waste heat in the order of kilowatts. This might not seem like much, but it is focused on the delicate, tiny internal optics that bend the beam into the reaction chamber. A failure of this system to provide enough coolant will lead the the propulsion system erupting into flames before melting away under the exponentially increasing temperatures.

Lightsails fail in less dramatic ways.

Large, flimsy, inefficient... but damn beautiful on screen.

To accelerate lightsails by any appreciable amount, you have to bounce off a huge amount of power off them. This translates into using very powerful lasers. If even a tiny portion of that energy goes into heating up the lightsail material instead of reflecting away, it would rapidly increase the temperature of the lightsail's surface. It would reach the point where chemical reactions are forced, which case it always becomes less reflective. The most reflective surfaces, dielectric mirrors, are also the most vulnerable to this. These damaged, less reflective surfaces absorb more heat, and in a feedback loop, burn away like paper under a magnifying lens.

The causes can be imperfections during manufacturing, rough handling of the flimsy material, or simply the ragged edges of a micrometeorite strike.

Chemical

Ah, the heady smell of kerosene, the acrid stink of dinitrogen tetroxide, the chill of liquid oxygen... makes us feel right at home!

Chemical rockets can be very simple or very complex, and their failures range from whimpers to sun-blotting explosions.

The simplest rockets use solid fuels. Oxidizer and reagent are all stacked together in a tower. If it goes right, it turns into a bone-rattling barely controlled burn that cannot be stopped. If it goes wrong, it turns into a bone-crushing uncontrolled explosion that cannot be stopped.

NASA solid rocket booster test.

Solid Rocket Boosters are some of the rare things that blow up in real life when you shoot them.

The biggest catastrophes happen when the chemicals start reacting in the wrong spot. This creates an impressive explosion. Even after you ignite it properly, and think you have control over it, things can go wrong. You see, after the lower-most fuels have been consumed, they start becoming the reaction chamber and nozzle for the rest of the booster. Between the vibrations and heat, the solid fuel's inner surface can fissure and flake off, creating spikes in reaction rate that translate into sudden accelerations and more vibrations. SRBs can literally shake themselves to pieces.

Another danger to look out for, especially due to manufacturing errors, is when the fire inside the solid rocket booster starts eating into the fuels in unplanned-for directions, at unplanned-for rates. This creates assymetrical forces that push against the walls diagonally, something they are not designed for. The result? It pops like a firecracker.

Liquid fuelled rockets are entirely different. They operate by piping huge amounts of liquid into a reaction chamber and hoping they explode together in just the right way.

Of course, things have been, and will, go wrong.

Antares rocket exploding.

Most problems appear right after ignition. The pumps start being fed by a gas generator, turning them at thousands of revolutions per minutes. They force the liquids contained in fragile, sub-zero tanks through tubes at incredible pressures, spraying them into a reaction chamber. There, they encounter a glorified cigarette lighter, which ignites them. The momentum of the fluids moves the fire downrange, where it bursts out of the nozzle. Within seconds, the nozzle is relying solely on a coolant flow to prevent melting.

If the tanks are damaged or broken, fluid will leak and warm up. If it's liquid hydrogen, it sticks around as an explosion hazard. If it's kerosene, it slowly dribbles down, heavy and sticky, until it reaches a naked flame or spark. When cryogenic fluids heat up too much, they simply burst out from their containers.

If the pumps don't work, or provide an uneven mix of fluids, the rocket fails. Recently, the Akatsuki probe's hypergolic fuel valve was found to be clogged. This made the mixture fuel-poor. It burned too hotly in the excessive oxidant, and cracked the ceramic nozzle.

The list goes on... pumps can shatter and their shrapnel pierces everything like bullets, pipes can burst and douse everything with fuel, the reaction chamber can overhead and crack, leaking spouts of flame, the nozzle can heat up and split open, creating uneven thrust and flipping the rocket around...

Part II will deal with propulsion systems from Electromagnetic to Pulse propulsion, passing through Nuclear thermal rockets.

7 comments:

Actually, there IS a second choice for handling antimatter. Gravitic manipulation, if part of the milieu (and when is it not part of an antimatter using milieu), provides a second method. Keep in mind, gravity manipulation is frequently linked to tractor and pressor beams; if those are not gravitic (and in a couple odd games, they aren't, being variations on magnetic technologies) then they are a third technology and point of failure, because they are the best no-touch regulator of how to move stuff inside a magnetic or antigravitic bottle. Further, Gravitics, especially the inertial compensation, doing unexpected things can potentially create forces that will drive them through the magnetic fields.

Another method for handling antimatter that researchers are toying with, with some level of success, is with laser piching. Interestingly, the lasers are actually used to COOL the antimatter down to near absolute zero. The lasers are then used to move the antimatter whereit is wanted, and to keep it there. I don't know how much energy these lasers require, but the tchnique is probably the best chance at collecting and storing usefully large masses of antimater and preventing loss of containment.

all of the mentioned failure modes for lightsails (thermal stability, misalignment) are addressed on the conical carbon sails proposed and studied by Jim Benford. The conical shape is self-aligning, which means that if the cone is slightly off the axis of the beam, a force gradient tends to move it back to the center. Carbon material also has very low area density, and can stand 3000K without flinching

-the microwave concept is great and cheap for short ranges, but terrible if you are trying to maintain a small spot size over longer ranges. The need for shorter wavelengths precludes the use of materials that rely on their heat resistance to copensate for lack of reflectivity.